1. Field of the Invention
This invention provides a structure for reducing the aerodynamic drag of a vehicle and a vehicle having reduced aerodynamic drag during highway operation. The invention is applicable primarily to trucks and semi-trucks that travel on the interstate highway system.
2. Brief Description of the Prior Art
Higher energy costs have focused attention on reducing the aerodynamic drag of trucks and semi-trucks. These vehicles typically travel interstate highways for long distances with payloads of 18,000 to 36,000 kg (20 to 40 tons) and at speeds of 100-130 km per hour (60 to 80 miles per hour).
The typical trailer of these vehicles has a squared-off rear end with large doors that pivot around vertical axes to fold back along the sides of the trailer, thereby enabling backing the trailer firmly against raised loading and unloading docks. This in turn enables forklifts and other powered vehicles to move into and out of the trailer readily to assist in loading and unloading.
But the squared-off rear end creates significant aerodynamic turbulence during operation at highway speeds, and the turbulence results in increased drag and increased fuel consumption. An inflatable streamlining apparatus for reducing the aerodynamic turbulence at the rear end of semi-trailers was proposed nearly fifty years ago in U.S. Pat. No. Potter 2,737,411. The apparatus included a flexible bag that was inflated during highway operation to a shape that minimized aerodynamic drag at the rear of a truck. During operation in close quarters and during loading and unloading, deflating the bag enabled built-in springs to roll it up against the rear doors of the trailer to minimize interference.
The Potter apparatus did not achieve commercial acceptance even though the benefits of reducing the aerodynamic drag of over-the-road vehicles were recognized and verified. Accordingly many improvements to inflatable streamlining structure like that taught by Potter were proposed and developed over the years. Improvements were directed at including internal and external belts to improve the shape of the flexible bag and achieve better aerodynamic efficiency, using vortex generators to smooth airflow over the bag, using ram air generated by vehicle motion to inflate the bag (and which was intended to deflate the bag automatically when the vehicle stopped in preparation for docking), including a system of ropes and pulleys to pull a deflated flexible bag into a storage position above or below the trailer, and including internal structural elements to provide a desired aerodynamic shape of the bag when in use on the highway and enable folding of the bag against the rear doors of the vehicle for low speed operation and docking.
Nonetheless, commercial acceptance of apparatus like that proposed by Potter and the resulting benefit of reduced fuel consumption have not been achieved.
This invention provides an improved structure for reducing the aerodynamic drag of a vehicle that is more efficient to inflate and deflate and is readily removable from an obstructing position on the vehicle doors to more readily facilitate low speed operation and docking. The structure comprises at least two layers of flexible material having peripheral portions and interior portions. The peripheral portions of each layer are pneumatically sealed to each other. Connecting and restraining means extend through the interior portions at multiple points and connect the interior portions of the layers to each other while restraining separation of the layers from each other for a controlled distance so that the interior portions of the layers are in a spaced relationship to each other. Air admitting means are attached to the layers for inflating the space between the interior portions of the layers to impose tension within the connecting and restraining means.
The structure is attached to a vehicle by attaching rope means to the peripheral portions, attaching channel means to the vehicle, and fitting the rope means into the channel means. Preparing the vehicle for highway operation involves a simple operation of inflating the interior portion of the layers to a suitable and relatively modest pressure. The layers and the connecting and restraining means are assembled so that the resulting structure is shaped when inflated to improve the aerodynamic flow at the end of a vehicle trailer. The combination of the layers, the restraining means, and the inflation pressure results in an essentially rigid and light weight structure having a shape that minimizes air turbulence at the rear end of the vehicle during highway operation and improves aerodynamic efficiency.
Preferred connecting and restraining means comprise inter-stitching the layers together in various patterns. The inter-stitching holds the interior portions of the layers in a spaced relationship wherein the interior portions of the layers are separated by twenty-five to seventy-five millimeters (one to three inches). A zig zag pattern with stitching frequency of one stitch per mm, the legs of the zig zags ten millimeters long, and the zig zags spaced by ten millimeters produces a structure that is particularly useful for interstate highway travel. The connecting and restraining means can include a fibrous material that partially fills the interior portions. The lengths of the inter-stitching can be varied along several lines of the interior portions to provide built-in ridges and grooves that assist in achieving and maintaining smooth, laminar air flow over the structure under the variable conditions encountered during highway operation.
The structure is lightweight but sufficiently rigid to maintain its design shape despite buffeting caused by cross winds and traffic effects. When the vehicle is operating at low speeds or is being prepared for docking, a simple operation of deflating the layers and either folding them against the doors of the trailer or removing them from the vehicle enables ready access to the interior of the trailer.
In a preferred embodiment particularly suited to semi trailers with left and right doors that swing open about vertical axes formed by hinges at each side of the rear end of the trailer, a structure of the invention is attached separately to each door. The peripheral portions of each structure that face each other when the doors are closed are essentially square and fit against each other as the rear doors of the trailer are closed. A zipper or velcro fastener attaches these peripheral portions to each other to form a continuous aerodynamic shape.